1. Field of the Invention
The invention relates to an electromagnetic clutch for controlling the transmission of torque between rotating members or the braking of a rotating member.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-17807 (JP-A-2004-17807), for example, describes a related electromagnetic clutch that includes an output mechanism that outputs moving force by generating electro-magnetic force, and a cam mechanism that operates by the driving of an electric motor on the axis of this output mechanism.
The output mechanism of the electromagnetic clutch has an electromagnetic coil that generates electro-magnetic force, and an armature that moves in response to the electromagnetic coil being energized. This output mechanism is arranged on a periphery of an output shaft.
The electromagnetic coil is housed in a coil housing that rotates together with the output shaft, and is fixed to the vehicle body side.
The armature is arranged in a position facing the electromagnetic coil via a bottom portion of the coil housing. Also, the armature is configured to friction engage with the coil housing in response to output from the output mechanism. Also, the armature is arranged so as to move away from the coil housing by the spring force of a return spring when the output mechanism stops outputting moving force.
The cam mechanism has a gear that rotates by the driving of the electric motor, and a cam follower interposed between the gear and the armature, and is arranged on the axis of the output mechanism.
The gear is arranged so as to be able to rotate around the periphery of the output shaft, and is coupled to an input shaft (i.e., a motor shaft of the electric motor) via a speed reducing gear train.
The cam follower is formed by a spherical member, and is rollably arranged between the gear (i.e., a cam groove therein) and the armature (i.e., a cam groove therein).
According to the structure described above, when the electromagnetic coil is in an energized state when the electric motor is being driven, the armature moves to the electromagnetic coil side and friction engages with the coil housing. Consequently, the cam mechanism operates. Therefore, the cam action from operation of the cam mechanism causes the armature to friction engage more strongly than before the cam mechanism is operated, such that the driving torque of the electric motor is transmitted via the cam mechanism and the like to the output shaft (i.e., to the differential side).
On the other hand, when the electromagnetic coil is in a de-energized state (i.e., not energized) when the electric motor is stopped, the cam mechanism does not operate, so friction engagement between the armature and the coil housing is canceled by the spring force of the return spring. As a result, the transmission of driving torque from the electric motor to the differential side is interrupted.
In the electromagnetic coil described in JP-A-2004-17807, if there is a sudden change in the rotation speed of the cam member when the electromagnetic coil is de-energized, the armature is unable to follow this sudden change in rotation speed because the armature tends to rotate from inertia, resulting in relative rotation between the cam member and the armature. As a result, the cam mechanism may operate erroneously, such that the armature ends up friction engaging with the coil housing when the electromagnetic coil is not energized. Therefore, in order to prevent this friction engagement, there is a need to restrict acceleration of the armature.